CN112713183B - Preparation method of gas sensor and gas sensor - Google Patents

Abstract

The invention provides a preparation method of a gas sensor and a gas sensor. The method includes: providing a Si substrate with a GaN buffer layer on the surface; forming an epitaxial stack on the GaN buffer layer, the epitaxial stack is formed from From bottom to top are GaN channel layer, AlN insertion layer, AlGaN barrier layer, and GaN cap layer in sequence; the epitaxial stack is etched to form a stepped heterostructure, and device isolation is achieved at the same time; ohmic contact electrodes and Schotts are formed base contact electrode, the ohmic contact electrode is located on the GaN cap layer, and the Schottky contact electrode is located on the GaN buffer layer at the bottom of the etched region; annealing; forming a sensitive material layer, the sensitive material layer is connected to the heterostructure and the Schottky contact electrodes. The present invention reduces the turn-on voltage and power consumption of the gas sensor. At the same time, the distance between the sensitive material layer and the two-dimensional electron gas is shortened, the transport of carriers no longer tunnels through the AlGaN barrier layer, the high mobility is maintained, and the device has higher sensitivity.

Description

Preparation method of gas sensor and gas sensor

technical field

The invention relates to the field of semiconductors, in particular to a preparation method of a gas sensor and a gas sensor.

Background technique

In recent years, gas sensors based on AlGaN/GaN heterojunction devices have gradually attracted extensive attention. Due to the high forbidden band width of GaN material and the existence of two-dimensional electron gas (2DEG) with high mobility at the AlGaN/GaN heterojunction interface and easily controlled by the surface state, gas sensors based on AlGaN/GaN heterojunction devices have good performance. High temperature stability and gas sensitivity. Therefore, such devices are suitable for real-time monitoring of toxic and harmful gases emitted from combustion or exothermic reaction systems in high temperature environments.

At present, the main structures of gas sensors based on AlGaN/GaN heterojunctions include Schottky diodes and field effect transistors. The former places the sensitive material at the Schottky contact end, and when the target gas contacts the sensitive material, a reversible redox reaction occurs, which changes the carrier density and energy band structure inside the sensitive material, causing the Schottky contact barrier to occur. changes, thereby changing the turn-on voltage and turn-on current of the Schottky diode. The latter places the sensitive material at the gate electrode. When the work function of the sensitive material changes, the field effect on the 2DEG at the heterojunction interface changes, which changes the carrier concentration at the channel, so the threshold voltage of the transistor and The on-current changes accordingly. In order to simplify the device structure, the gate structure is often chosen to be omitted in the actual preparation of AlGaN/GaN heterojunction transistors, and the sensitive material directly controls the output current between the source and drain electrodes. Due to the large power loss in the static state of the FET structure, the current research focuses on gas sensors based on AlGaN/GaN heterojunction Schottky diodes with a certain turn-on voltage.

Among them, the key factors that determine the performance of AlGaN/GaN heterojunction Schottky diode gas sensors are the preparation of sensitive materials and the design of the overall structure of the device. At present, the two most common types of sensitive materials are noble metal thin films such as Pt and Pd and metal oxide nanostructures such as ZnO and SnO ₂ . In recent years, there have been many new research reports in the preparation of sensitive materials, such as the use of nanoparticle decoration, nanostructures with high specific surface area such as nanosheets or nanoflowers, and the use of different oxide heterojunctions to enhance sensing performance. However, no matter how the sensitive material changes, the sensitive material is in contact with the AlGaN barrier layer or the GaN cap layer on the Schottky contact side, which is always in contact with the AlGaN/GaN heterojunction surface, which limits the device sensing performance to some extent. First, due to the existence of the AlGaN barrier layer, the device has a high turn-on voltage, and the device often needs to work at a voltage of 0.5V and above, which increases the power consumption of the device. Second, electrons pass through the AlGaN barrier layer through the tunneling effect, which reduces the carrier mobility and limits the sensitivity of the sensing device. Based on the above two points, it is necessary for gas sensors based on AlGaN/GaN heterojunction Schottky diodes to improve the traditional device structure and further improve the device performance.

SUMMARY OF THE INVENTION

The technical problems to be solved by the present invention are the problems of high working voltage, large power consumption and unsatisfactory sensitivity of the gas sensor, and a preparation method of the gas sensor and a gas sensor are provided.

In order to solve the above problems, the present invention provides a preparation method of a gas sensor, which includes: providing a Si substrate with a GaN buffer layer on the surface; forming an epitaxial stack on the GaN buffer layer, the epitaxial stack from the bottom Upwards are the GaN channel layer, the AlN insertion layer, the AlGaN barrier layer, and the GaN cap layer; the epitaxial stack is etched to form a stepped heterostructure, and device isolation is achieved at the same time; the ohmic contact electrode and Schottky are formed contact electrode, the ohmic contact electrode is located on the GaN cap layer, and the Schottky contact electrode is located on the GaN buffer layer at the bottom of the etched region; annealing; forming a sensitive material layer, the sensitive material layer is connected to the isolating material structure and the Schottky contact electrode.

In order to solve the above problems, the present invention also provides a gas sensor, comprising: a Si substrate with a GaN buffer layer on the surface; a heterostructure, the heterostructure is located on the GaN buffer layer, and the order from bottom to top is GaN channel layer, AlN insertion layer, AlGaN barrier layer, and GaN cap layer; an ohmic contact electrode on the GaN cap layer of the heterostructure; a Schottky contact electrode, the ohmic contact electrode A Teky contact electrode is located on the GaN buffer layer; a sensitive material layer connects the heterostructure and the Schottky contact electrode.

In the gas sensor disclosed in the present invention, the transport of carriers is no longer limited by the Schottky barrier between the sensitive material layer and the AlGaN barrier layer, but is transformed into the surface potential barrier between the sensitive material and the AlGaN/GaN heterostructure, The turn-on voltage and power loss of the device are reduced. At the same time, the structure shortens the distance between the sensitive material layer and the two-dimensional electron gas, and the transport of carriers no longer tunnels through the AlGaN barrier layer, maintaining high mobility, and the device has higher sensitivity.

Description of drawings

FIG. 1 is a schematic diagram of the steps described in a specific embodiment of the present invention.

2A-2G are schematic diagrams of the processes of steps S10-S15 in FIG. 1 .

Detailed ways

The method for preparing a gas sensor and the specific implementation of the gas sensor provided by the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a schematic diagram of the steps according to a specific embodiment of the present invention, including: step S10, providing a Si substrate with a GaN buffer layer on the surface; step S11, forming an epitaxial stack on the GaN buffer layer, so The epitaxial stack is, from bottom to top, a GaN channel layer, an AlN insertion layer, an AlGaN barrier layer, and a GaN cap layer; in step S12, the epitaxial stack is etched to form a stepped heterostructure, and device isolation is achieved at the same time Step S13, forming an ohmic contact electrode and a Schottky contact electrode, the ohmic contact electrode is located on the GaN cap layer, and the Schottky contact electrode is located on the GaN buffer layer at the bottom of the etched region; Step S14, annealing ; Step S15, forming a sensitive material layer, the sensitive material layer connecting the heterostructure and the Schottky contact electrode.

As shown in FIG. 2A , referring to step S10 , a Si substrate 201 with a GaN buffer layer 202 on the surface is provided. In a specific embodiment of the present invention, the thickness of the Si substrate 201 is 400 μm, and the thickness of the GaN buffer layer 202 is 3 μm. The material of the Si substrate 201 can also be replaced with common substrate materials in the semiconductor field, such as sapphire, silicon carbide, and gallium nitride.

As shown in FIG. 2B , referring to step S11 , an epitaxial stack 21 is formed on the GaN buffer layer 202 , and the epitaxial stack 21 is a GaN channel layer 203 , an AlN insertion layer 204 , and an AlGaN barrier layer in order from bottom to top. 205 , and a GaN cap layer 206 . In a specific embodiment of the present invention, the thickness of the GaN channel layer 203 is 500 nm, the thickness of the AlN insertion layer 204 is 1 nm, the thickness of the AlGaN barrier layer 205 is 20 nm, and the thickness of the GaN cap layer The thickness of 206 is 3 nm.

As shown in FIG. 2C , referring to step S12 , the epitaxial stack 21 is etched to form a stepped heterostructure 22 , and device isolation is achieved at the same time. In a specific embodiment of the present invention, the heterostructure 22 is an AlGaN/GaN heterostructure, which is formed by etching in a mixed gas of BCl ₃ and Ar through processes such as photolithography and etching, and has a height of 0.2 -1μm.

As shown in FIG. 2D, referring to step S13, an ohmic contact electrode 207 and a Schottky contact electrode 208 are formed, the ohmic contact electrode 207 is located on the GaN cap layer 206, and the Schottky contact electrode 208 is located in the etching region on the GaN buffer layer 202 at the bottom. In a specific embodiment of the present invention, the stacked structure of the ohmic contact electrode 207 and the Schottky contact electrode 208 is formed by processes such as photolithography, electron beam evaporation, etc.: a metal Ti layer with a thickness of 20 nm, a thickness of A metal Al layer with a thickness of 120 nm, a metal Ni layer with a thickness of 30 nm, and a metal Au layer with a thickness of 50 nm are formed.

Annealing, in a specific embodiment of the present invention, adopts a rapid annealing process, the annealing temperature is 800-900° C., and the annealing time is 30-60 s. After annealing, the ohmic contact electrode 207 is in ohmic contact with the two-dimensional electron gas; the Schottky contact electrode 208 is in contact with the etched bottom GaN buffer layer 202 and is not electrically connected with the two-dimensional electron gas.

As shown in FIG. 2E , referring to step S15 , a sensitive material layer 209 is formed, and the sensitive material layer 209 connects the heterostructure 22 and the Schottky contact electrode 208 . The sensitive material layer 209 is prepared by using gas sensitive materials, and can be prepared by using metal materials and semiconductor metal oxide materials.

As shown in FIG. 2F , in an embodiment of the present invention, the sensitive material layer adopts a metal Pt layer 210 . The process of preparing the metal Pt layer 210 is as follows: depositing the metal Pt layer 210 in the etched step region by photolithography, electron beam evaporation, etc., and contacting the AlGaN/GaN heterostructure 22 and the Schottky contact electrode 208 at the same time.

As shown in FIG. 2G , in another specific embodiment of the present invention, the sensitive material layer adopts a ZnO nanowire array 211 . The process of preparing the ZnO nanowires is further as follows: using the atomic layer deposition technology to grow the ZnO thin film seed layer on the surface of the GaN buffer layer 202 and the heterostructure 22, and each cycle growth process of the atomic layer deposition process includes 0.2s of diethylzinc ( DEZ) pulse, 2s _N2 purge, 0.2s deionized water pulse, 2s N2 purge, ALD growth temperature was 200°C, ZnO film rate was 0.1nm/cycle, ZnO seed layer thickness was 10-20nm ; Remove the ZnO seed layer outside the area of the sensitive material layer to be grown by processes such as photolithography, wet etching, etc. The wet etching process is to use BOE solution (40%HF: ₄₀ %NH4F=1:6) or hydrochloric acid solution (HCl:H ₂ O=1:10), the etching time is 5s; the area outside the sensitive material layer area is protected with photoresist by the photolithography process, and only the area where the ZnO seed layer is grown is exposed, and then the The ZnO nanowires 211 are grown hydrothermally, and finally the photoresist is removed to form the final device structure. The ZnO nanowires 211 are connected to the AlGaN/GaN heterostructure 22 and the Schottky contact electrode 208 . The precursor solution used in the hydrothermal growth is a mixed solution of 25mM Zn(NO ₃ ) ₂ ·6H ₂ O and 25mM hexamethyltetramine (HMT), the growth temperature is 80-90°C, and the growth time is 4-8h.

Next, a specific implementation manner of a gas sensor obtained after the above steps are implemented will be given in conjunction with the accompanying drawings. The structure of the gas sensor is shown in FIG. 2E, including: a Si substrate with a GaN buffer layer 202 on the surface 201; a heterostructure 22, the heterostructure 22 is located on the GaN buffer layer 202, and is a GaN channel layer 203, an AlN insertion layer 204, an AlGaN barrier layer 205, and a GaN cap layer 206 in order from bottom to top; a Ohmic contact electrode 207, the ohmic contact electrode 207 is located on the GaN cap layer 206 of the heterostructure 22; a Schottky contact electrode 208, the Schottky contact electrode 208 is located on the GaN buffer layer 202; A sensitive material layer 209 connecting the heterostructure 22 and the Schottky contact electrode 208 .

In the gas sensor disclosed by the above technical solution, the transport of carriers is no longer limited by the Schottky barrier between the sensitive material layer and the AlGaN barrier layer, but is transformed into the surface potential barrier between the sensitive material and the AlGaN/GaN heterostructure , reducing the turn-on voltage and power loss of the device. At the same time, the structure shortens the distance between the sensitive material layer and the two-dimensional electron gas, and the transport of carriers no longer tunnels through the AlGaN barrier layer, maintaining high mobility, and the device has higher sensitivity.

The above are only the preferred embodiments of the present invention. It should be pointed out that for those skilled in the art, without departing from the principles of the present invention, several improvements and modifications can also be made, and these improvements and modifications should also be regarded as It is the protection scope of the present invention.

Claims (7)

1. a preparation method of gas sensor, is characterized in that, comprises: providing a Si substrate with a GaN buffer layer on the surface; forming an epitaxial stack on the GaN buffer layer, the epitaxial stack is a GaN channel layer, an AlN insertion layer, an AlGaN barrier layer, and a GaN cap layer in order from bottom to top; The epitaxial stack is etched to form a stepped heterostructure, and device isolation is achieved at the same time; an ohmic contact electrode and a Schottky contact electrode are formed, the ohmic contact electrode is located on the GaN cap layer, and the Schottky contact the electrode is on the GaN buffer layer at the bottom of the etched region and is not in direct contact with the heterostructure; annealing; A sensitive material layer is formed, the sensitive material layer covers the step region of the heterostructure and connects the heterostructure and the Schottky contact electrode. 2 . The method according to claim 1 , wherein the stepped heterostructure is formed by etching a mixed gas of BCl ₃ and Ar, and the height is 0.2-1 μm. 3 . 3 . The method according to claim 1 , wherein the ohmic contact electrode and the Schottky contact electrode are in a stacked structure, and are composed of a metal Ti layer with a thickness of 20 nm and a metal Al layer with a thickness of 120 nm. 4 . , a metal Ni layer with a thickness of 30 nm, and a metal Au layer with a thickness of 50 nm. 4 . The method according to claim 1 , wherein the annealing temperature is 800-900° C., and the annealing time is 30-60 s; after annealing, the ohmic contact electrode forms ohmic contact with the two-dimensional electron gas. 5 . 5 . The method according to claim 1 , wherein the sensitive material layer is prepared by using a gas sensitive material, which can be prepared by using a metal material and a semiconductor metal oxide material. 6 . 6. The method according to claim 1, wherein the sensitive material layer is a Pt metal layer or a ZnO nanowire array. 7. A gas sensor, characterized in that, comprising: A Si substrate with a GaN buffer layer on the surface; A heterostructure, the heterostructure is located on the GaN buffer layer, from bottom to top, the GaN channel layer, the AlN insertion layer, the AlGaN barrier layer, and the GaN cap layer; an ohmic contact electrode, the ohmic contact electrode on the GaN cap layer of the heterostructure; a Schottky contact electrode on the GaN buffer layer and not in direct contact with the heterostructure; a sensitive material layer covering the step area of the heterostructure and connecting the heterostructure and the Schottky contact electrode.

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